Rhamnolipids (RL), an interface-active glycolipid type biosurfactant, having the properties to lower the surface tension between two different liquids are broadly used as emulsifier, detergents and foaming agents [1-3]. Rhamnolipids are shown to be able to efficiently remove crude oil from contaminated soil and facilitate bioremediation of oil spills due to their emulsification properties which so called “enhanced oil recovery (EOR)” [4]. They have also been used in agriculture due to their antimicrobial and antifungal properties. Rhamnolipids are currently available in an agricultural anti-fungal product marketed as ZONIX and in an industrial cleaning product, RECO, for cleaning oil from storage tanks.
The genus Pseudomonas and E. coli are shown to be capable of producing rhamnolipid (RL) from a variety of carbon and nitrogen sources [5]. Although, the genus Pseudomonas produces higher RL yields and titers than the E. coli [5, 6], the RL concentration is not high enough to make the fermentation process commercially viable. A number of approaches have been carried out to increase the RL productivity including different carbon sources, genetic modified strains and fermentation strategy which appears to be the most effective route to achieve.
Batch and fed batch fermentation are the most common fermentation process with Pseudomonas [7-11]. Batch fermentation is the simplest cultivation method [7,8] in which feedstock (carbon source) is added in the fermenter at the beginning with the inoculum. The fermentation takes place until the feedstock is all utilized by the microorganisms, then the fermenter is shut down and the new batch is started [9]. Fed batch, on the other hand, is batch fermentation but with the addition of feedstock (carbon source), after the feedstock is fully utilized, over the course of fermentation [9-11]. The best up-to-date reported RL titer (productivity) is about 0.58-0.72 g/L/h obtained from fed-batch fermentation of P. aeruginosa [10]. However, this process requires the shutdown of fermenter after 90-120 hours prior to starting up a new fed batch. Similarly, U.S. Pat. No. 5,501,966 reported a 78 g/L of rhamnolipids produced after 167 hours [8] which is too long to operate.
TABLE 1Summary of various rhamnolipid (RL) fermentationprocesses by P. aeruginosaFermenta-RLFermenta-RLtion timetiterRefer-tion type(g/L)(h)(g/L/h)enceBatch701440.49 [7]Batch781670.47 [8]Fed batch65900.72[10]Fed batch701200.58[10]Fed batch4.1720.06[11]Solid state462880.16[12]